2021-11-11 07:52:58

Traveling at Warp 1

Announcements

  • Exam 3 Tuesday, 11/16
    • On Canvas, 3:05 PM - 10:00 PM

Today’s topics

Vision

How vision informs

  • What’s out there?
    • Shape, form, color
  • Where is it?
    • Position, orientation, motion

Electromagnetic (EM) radiation

Features of EM radiation

  • Wavelength or frequency
  • Intensity
  • Location/position of source
  • Reflects off some materials
  • Refracted (bent) moving through other materials

EM radiation provides information across space (and time)

Reflectance spectra differ by surface

Reflectance spectra

Optic array specifies geometry of environment

Color == categories of wavelength

  • Eyes categorize wavelength into relative intensities within wavelength bands
  • RGB ~ Red, Green, Blue
    • Long, medium, short wavelengths
  • Color is a neural/psychological construct

RGB monitors

How a camera works

The biological camera

The biological camera

Parts of the eye

  • Cornea - refraction (2/3 of total)
  • Pupil - light intensity; diameter regulated by the Iris.
  • Lens - refraction (remaining 1/3; variable focus)

Parts of the eye

  • Retina - light detection
    • ~ skin or organ of Corti
  • Pigment epithelium - regenerate photopigment
  • Muscles - move eye, reshape lens, change pupil diameter

Eye forms image on retina

  • Image inverted (up/down)
  • Image reverseed (left/right)
  • Point-to-point map (retinotopic)
  • Binocular and monocular zones

Retinal image

Eyes views overlap

The fovea

The fovea

  • Central 1-2 deg of visual field
    • ~ thumbnail @ arm’s length
  • Aligned with visual axis; center of gaze
  • Retinal ganglion cells pushed aside
  • Highest acuity vision == best for details

Acuity varies across the retina

Acuity varies across the retina

What part of the skin is like the fovea?

Photoreceptors detect light

Photoreceptors detect light

  • Rods
    • ~120 M/eye
    • Mostly in periphery
    • Active in low light conditions
    • One wavelength range

Photorceptors detect light

  • Cones
    • ~5 M/eye
    • Mostly in center
    • 3 wavelength ranges

Photoreceptors “specialize” in particular wavelengths

How photoreceptors work

  • Outer segment
    • Membrane disks
    • Photopigments
      • Sense light, trigger chemical cascade
  • Inner segment
    • Synaptic terminal
  • Light hyperpolarizes photoreceptor!
    • The dark current

Retina

  • Physiologically backwards
    • How?
  • Anatomically inside-out
    • How?

Retina

  • Physiologically backwards
    • Dark current (more NT released in dark)
  • Anatomically inside-out
    • Photoreceptors at back of eye

Retinal layers

Retinal layers

  • Bipolar cells
    • Horizontal cells
  • Retinal ganglion cells
    • Amacrine cells

Center-surround receptive fields

Center-surround receptive fields

  • Center region
    • Excites (or inhibits)
  • Surround region
    • Does the opposite
  • Bipolar cells & Retinal Ganglion cells ->
  • Most activated by “donuts” of light/dark
    • Local contrast (light/dark differences)

What’s a reddish-green look like?

What’s a reddish-green look like?

Opponent processing

Opponent processing

  • Black vs. white (achromatic)
  • Long ( red) vs. Medium ( green) wavelength cones
  • (Long + Medium) vs. Short ( blue) cones
  • Can’t really see reddish-green or bluish-yellow

From eye to brain

From eye to brain

  • Retinal ganglion cells
  • 2nd/II cranial (optic) nerve
    • Optic chiasm

From eye to brain

  • Hypothalamus
    • Suprachiasmatic nucleus
      • Regulates circadian (day/night) rhythm via pineal gland

From eye to brain

  • Superior colliculus & brainstem

Lateral Geniculate Nucleus (LGN) of thalamus

  • ~90% of axons from retina

References